Portable products, such as personal computers, digital cameras, and portable phones, have recently prevailed and have been demanded to have small sizes, low profiles, small weights, high densities, and multifunction. According to this demand, semiconductors have their packages having small sizes, low profiles, and three-dimensional mounting. A cavity board is known as a method of easily providing a semiconductor package having a low profile and three-dimensional mounting.
A conventional cavity board will be described below with reference to FIG. 24. FIG. 24 is a sectional view of a conventional solid printed circuit board having a cavity. In FIG. 24, lower printed circuit board 22 and upper printed circuit board 23 are stacked across bonding layer 21 between the boards while electrodes and windows are positioned. Then, circuit boards 22 and 23 and bonding layer 21 are heated and pressed to provide multilayer printed circuit board 27 having a cavity for inserting an electronic component. Patent document 1 is known as a prior art document related to the present invention.
The conventional multilayer printed circuit board shown in FIG. 24 generally uses a prepreg as the bonding layer. Prepregs contain core materials, such as woven fabrics, unwoven fabrics, or films, hence effectively holding shapes of via-conductors made of conductive paste. The prepregs prevent wirings thereon from being embedded in the upper and lower printed circuit boards. Bonding sheets, instead of the prepregs as the bonding layer, allows the wirings to be embedded therein, but cause the conductive paste filled in the bonding layer to flow. This problem prevents complete interconnections between circuit boards having a lot of pins, and reduces a wiring density within the circuit boards.
Patent Document 1: JP 2004-253774A